Poling a polymeric film containing dopant chromophore molecules or side-chain groups with high molecular hyperpolarizabilities (.beta.) has become a widely used strategy for preparation of films with large second order optical nonlinearity exhibiting properties such as second harmonic generation (SHG) and electro-optic modulation. The usual method of film preparation involves applying a strong external electrostatic field (up to 10.sup.6 V/cm.) to a doped film heated to above its glass transition temperature (T.sub.g), leading to a preferential alignment of dipolar chromophore groups along the field direction. Cooling to room temperature (below T.sub.g) prior to removal of the field enables a "freezing in" of the net polar alignment. In practice, some residual molecular mobility is often retained, leading to a loss of the polar alignment over a period of time (ranging from minutes to months) which depends on both polymer matrix and chromophore properties.
Second order nonlinear optical processes such as SHG are described quantitatively by the second order susceptibility tensor X.sup.(2). Under the electric dipole approximation, a second-order polarization, P.sup.2w is induced by the incident (optical) electric field E.sup.w according to: EQU P.sup.2w i=X.sup.(2) ijk E.sup.w j E.sup.w k (1).
The observed second harmonic generation efficiency is proportional to the square of P.sup.2w. From symmetry considerations, it is readily apparent from equation (1) that in a centrosymmetric medium all components Xijk=0. Furthermore, in media symmetric about a plane i=0 all X.sup.(2) components containing the index i either one or three times must similarly be zero. Thus in the case of a polymer poled by an electric field along the x direction as discussed above, the only nonzero components are: EQU X.sup.(2) xxx, X.sup.(2) xyy=X.sup.(2) xzz, and X.sup.(2) yxy=X.sup.(2) zxz.
Such relationships between the various X.sup.(2) components of poled polymers have been found in various studies (see, e.g., Williams, D. J., Angew. Chem. Int., 1984, 23:690, and Esselin, S. et al., SPIE Conf. Proc., 1988, 971:120). It will be shown below, in accordance with one aspect of the present invention, how polymers can be poled so as to obtain additional nonzero X.sup.(2) components, arising from asymmetry not only along the poling direction (the x direction) but also in a direction z, which is perpendicular to the poling direction.